1. Field of the Invention
The present invention relates to integrated circuits and, more particularly, to a method of preparing semiconductor wafers having die with copper pads to allow for more reliable wire bonding.
2. Background of Related Art
An integrated circuit (IC) die is a small device cut from a semiconductor wafer, such as a silicon wafer, on which multiple die have been formed. Such die are typically packaged to protect them from corrosion by attaching them to lead frames using a solder or epoxy. A die is electrically connected to leads in the lead frame, and then the die and the lead frame are encapsulated in a plastic package. The leads of the lead frame protrude from the package and terminate in pins that allow the die to be electrically connected with other circuits, such as on a printed circuit board.
Referring to FIG. 4, a conventional process for packaging a die is shown. First, at step 40, a die is cut or sawn from the wafer on which it has been formed. After the die has been cut from the wafer, the back side of the die is firmly attached to a carrier or lead frame in a die bonding or die attach step 42. Typically, in the die bonding step 42, the die is attached to the lead frame using an organic adhesive, such as an epoxy and then cured by baking. Once the epoxy has cured, in step 44 the die is wire bonded to the lead frame.
FIG. 5 is an enlarged cross-sectional view of a packaged integrated circuit 50. The packaged circuit 50 includes a die 51 bonded to a die attach pad 52. The die 51 is connected to a lead frame 53 by wires 54. Further, the die 51, die attach pad 52, wires 54 and part of the lead frame 53 are encapsulated or molded in a package 55. The package 55 may be plastic, metal, ceramic or another known packaging material.
The wires 54 connect bonding pads of the die 51 to bonding pads on the lead frame 53. The most common die-connection technology is wire bonding. Wire bonding is a solid phase welding process where two metallic materials, a very thin wire and a pad surface, are brought into contact. Once the surfaces are in contact, a combination of heat, pressure and/or ultrasonic energy is used to cause electron sharing or inter-diffusion of atoms, resulting in the formation of a wire bond.
Wire bonding is typically done using one of three industry standard techniques: thermo-compression (T/C) bonding, which uses a combination of pressure and elevated temperature; thermo-sonic (T/S) bonding, which uses a combination of pressure, elevated temperature, and ultrasonic vibration bursts; and ultrasonic (U/S) bonding, which uses a combination of pressure and ultrasonic vibration bursts. These wire bonding techniques are well known. The preferred bond wire material is gold, although other materials are also used, such as silver, aluminum/silicon, aluminum/magnesium, palladium, and copper.
Referring again to FIG. 4, after the wire bonding step 44, the back side of the die is cleaned, typically by ultraviolet-ozone (UVO) cleaning. In UVO cleaning, a UV-ozone cleaner that emits significant amounts of radiation is used to remove organic contaminants from the die. Finally, the die and lead frame assembly is molded, step 48, forming the packaged circuit 50 shown in FIG. 5.
In the last few years, there has been a renewed interest in using copper wire, as opposed to aluminum, in ICs due to the desire for higher clock rates and improved thermal management, as well as the ability to perform fine pitch and ultra-fine pitch copper wire bonding. In order to prevent intermetallic phases, it is preferred to bond copper wires to copper pads.
Unfortunately, copper tends to oxidize and corrode fairly quickly. Corrosion can open one or both ends of the wire bond completely, allowing the wire to move within the package, thereby causing electrical short circuits. The corrosion occurs in the presence of moisture and contaminants. For example, the presence of chlorine or bromine at the bonding area can cause the formation of chlorides or bromides, leading to bond corrosion. Bond corrosion also increases the electrical resistance of the wire bond interconnect. Thus, forming a reliable copper-to-copper bond can be difficult. Accordingly, it would be advantageous to have a pad surface to which wires may be bonded more reliably.
In order to provide more reliable wire bonds, the present invention provides a method of preparing a semiconductor wafer having a plurality of integrated circuits formed thereon, the integrated circuits having pads formed of copper. In the method, oxide is removed from the copper pads and then the wafer is vacuum packed in a shock-proof container. The oxide may be removed from the copper pads in a number of ways. A first way includes cleaning the wafer in an alkaline solution, performing acid neutralization on the cleaned wafer, and then drying the wafer. A second way includes cleaning the wafer with an acid solution, such as by dipping the wafer in a bath of H2SO4 or HNO3, rinsing the acid cleaned wafer with water, applying an anti-oxidant activator to the surface of the copper pads, rinsing the wafer with water after the application of the anti-oxidant activator, and then drying the water rinsed wafer. Yet a third way includes plasma cleaning the copper pads using about 5-10% Hydrogen and about 90-95% Argon in an ultra low vacuum pressure of about 5-20 mTorr and then sputtering a very thin layer of oxide passivation, such as aluminum on a surface of the copper pads. The layer of oxide passivation has a thickness on the order of about 1 to about 10 nanometers.
The present invention further provides a method of electrically connecting a copper pad of an integrated circuit with a pad of a lead frame with a copper wire, including the steps of plasma cleaning the copper pad using about 5-10% Hydrogen and about 90-95% Argon and then wire bonding the wire to the cleaned copper pad and the lead frame pad.